David B. Kittelson

Engines and Nanoparticles: A Review

Most of the particle number emitted by engines is in the nanoparticle range, Dp<50 nm, while most of the mass is in the accumulation mode, nm, range. Nanoparticles are typically hydrocarbons or sulfate and form by nucleation during dilution and cooling of the exhaust, while accumulation mode particles are mainly carbonaceous soot agglomerates formed directly by combustion. Emission standards on diesel engines have led to dramatic reductions in particle mass emitted. However, a new HEI study shows that some low-emission diesel engines emit much higher concentrations of nanoparticles than older designs and other low-emission designs. Many recent studies suggest that at similar mass concentrations; nanometer size particles are more dangerous than micron size particles. This has raised questions about whether nanoparticle (number based) emission standards should be imposed. Unlike mass, number is not conserved. It may change dramatically by nucleation and coagulation during dilution and sampling, making it very difficult to design a standard. Furthermore, if nanoparticles are a problem, spark ignition engines may also have to be controlled.

Generating Particle Beams of Controlled Dimensions and Divergence: I. Theory of Particle Motion in Aerodynamic Lenses and Nozzle Expansions

A particle beam is produced when a particle-laden gas expands through a nozzle into a vacuum. This work discusses the theoretical basis of a novel method for producing highly collimated and tightly focused particle beams. The approach is to pass the particle-laden gas through a series of axisymmetric contractions and enlargements (so-called aerodynamic lenses) before the nozzle expansion. Particles are moved closer to the axis by a lens if the particle sizes are less than a critical value and particles can be confined very closely to the axis by using multiple lenses in series. Since particles close to the axis experience small radial drag forces, they stay close to the axis during nozzle expansion and therefore form a narrow particle beam downstream. The major effects that limit the minimum beam width are Brownian motion and lift forces on particles during the nozzle expansion. Simple theoretical models are developed in this work to estimate the minimum particle beam width set by these effects. While the...

Generating Particle Beams of Controlled Dimensions and Divergence: II. Experimental Evaluation of Particle Motion in Aerodynamic Lenses and Nozzle Expansions

A particle-beam-forming apparatus for producing narrow particle beams was developed based on the theory discussed in paper I of this series. It consists of a variable number of aerodynamic lenses (short capillaries and/or thin-plate orifices with diameters ranging from 3.5 to 7.0 mm) followed by an accelerating nozzle (3 mm). It was evaluated using monodisperse DOS and NaCl particles (0.02–0.24 μm) at upstream pressures on the order of 1 torr. The particle beams produced by the lens-nozzle system were focused through a skimmer (1 mm) into a high vacuum chamber (10−4–10−5 torr) where the beam widths, velocities and transport efficiencies were measured. The experiments showed that as more lenses were added the particle beam widths were reduced asymptotically to the minimum values. For spherical particles (DOS) these minimum values are in good agreement with the Brownian limit derived in paper I. For nonspherical particles (NaCl) these minimum widths are much larger than the Brownian limit, indicating that b...

On-line measurements of diesel nanoparticle composition and volatility

Abstract A thermal desorption particle beam mass spectrometer (TDPBMS) and tandem differential mobility analyzers (TDMA) were used for on-line measurements of the chemical composition and volatility of nanoparticles and larger particles emitted from a modern, heavy-duty diesel engine operated at light and medium loads under laboratory conditions. Temperature-dependent TDPBMS mass spectra and mass spectra obtained using spectrally distinctive oil and synthetic Fischer–Tropsch fuel were analyzed using mass spectral matching methods to obtain quantitative information on the contributions of fuel, oil, oxidation products, and sulfuric acid to particle composition. TDMA measurements of volatility yielded information on nanoparticle vapor pressures and therefore on the composition of organic components. The results indicate that, for these operating conditions, the volatile component of both diesel nanoparticles and larger particles is comprised of at least 95% unburned lubricating oil. TDMA volatility measurements also detected residual species a few nanometers in diameter, which may be non-volatile cores (soot, metal oxide) or low-volatility organic compounds. These on-line analyses provide new insights into the mechanisms of diesel nanoparticle formation.

Structural Properties of Diesel Exhaust Particles Measured by Transmission Electron Microscopy (TEM): Relationships to Particle Mass and Mobility

Structural properties of diesel particles preclassified by particle mobility and mass are measured using transmission electron microscopy (TEM). These measurements enable us to determine the dynamic shape factor and inherent material density of diesel particles. We also compare fractal dimensions obtained independently using two different approaches. We show that the projected area equivalent diameter of mobility-classified diesel particles nearly equals the mobility diameter in the size range reported here (50 to 220 nm). Evidence for doubly charged particles and possible “fragments” are observed for DMA-classified particles on TEM substrates. The fractal dimension was obtained using two independent approaches. Images obtained by TEM were analyzed to determine the maximum length and the number of primary particles. The fractal dimension obtained from these measurements, D fL, was 1.75. The fractal dimension obtained from the mass-mobility relationship, D fm was 2.35. We found that these values are in reasonable agreement after accounting for the relationship between the projected area diameter and maximum length. The size-dependent dynamic shape factor and inherent material density of diesel particles are obtained from independent measurements of mobility (DMA), mass (APM), and volume (TEM). We found that the dynamic shape factor increased from 1.11 to 2.21, and that the inherent material density increased from 1.27 to 1.78 g/cm3 as particle mobility size increased from 50 to 220 nm. The increase in dynamic shape factor with size occurs because large particles are more irregular than smaller ones. The increase in density occurs because the ratio of elemental carbon to condensed organics increases with increasing size.

The aerosol mobility chromatograph: A new detector for sulfuric acid aerosols

Abstract A new instrument has been developed for measuring sulfuric acid aerosols. The instrument is called an Aerosol Mobility Chromatograph since it is based on the electrical mobility of aerosol particles and operates in a way similar to that of the conventional liquid or gas Chromatograph. The particle diameter range of the instrument is from 0.005 to 0.2 μm and the sensitivity (for detecting monodisperse sulfuric acid aerosols), from 0.01 to 10 −5 μm −3 , depending upon the specific particle detector used. This paper describes the operating principle of the AMC and the performance characteristics of a prototype device developed at the Particle Technology Laboratory, University of Minnesota.

Diesel Aerosol Sampling in the Atmosphere

The University of Minnesota Center for Diesel Research along with a research team including Caterpillar, Cummins, Carnegie Mellon University, West Virginia University (WVU), Paul Scherrer Institute in Switzerland, and Tampere University in Finland have performed measurements of Diesel exhaust particle size distributions under real-world dilution conditions. A mobile aerosol emission laboratory (MEL) equipped to measure particle size distributions, number concentrations, surface area concentrations, particle bound PAHs, as well as CO 2 and NO x concentrations in real time was built and will be described. The MEL was used to follow two different Cummins powered tractors, one with an older engine (L10) and one with a state-of-the-art engine (ISM), on rural highways and measure particles in their exhaust plumes. This paper will describe the goals and objectives of the study and will describe representative particle size distributions observed in roadway experiments with the truck powered by the ISM engine.

Physical and Chemical Properties of Bio-Oils From Microwave Pyrolysis of Corn Stover

This study was aimed to understand the physical and chemical properties of pyrolytic bio-oils produced from microwave pyrolysis of corn stover regarding their potential use as gas turbine and home heating fuels. The ash content, solids content, pH, heating value, minerals, elemental ratio, moisture content, and viscosity of the bio-oils were determined. The water content was approx 15.2 wt%, solids content 0.22 wt%, alkali metal content 12 parts per million, dynamic viscosity 185 mPa·s at 40°C, and gross high heating value 17.5 MJ/kg for a typical bio-oil produced. Our aging tests showed that the viscosity and water content increased and phase separation occurred during the storage at different temperatures. Adding methanol and/or ethanol to the bio-oils reduced the viscosity and slowed down the increase in viscosity and water content during the storage. Blending of methanol or ethanol with the bio-oils may be a simple and cost-effective approach to making the pyrolytic bio-oils into a stable gas turbine or home heating fuels.

Characterization of Aerosol Surface Instruments in Transition Regime

The primary purpose of this study is to measure the size- and composition-dependent responses of aerosol surface instruments designed to measure surface area related properties. Measurements were conducted in the range of 30–150 nm of mobility equivalent diameter, Dp. The responses of a LQ1-DC (a diffusion charger manufactured by Matter Engineering AG) and an EAD (a diffusion charger manufactured by TSI) to singlets (NaCl) particles are proportional to Dp 1.36 and Dp 1.13, respectively. The response of LQ1-DC agrees with Fuchs surface area, which is proportional to Dp 1.39 within 2.4% error. The response of the EAD is almost proportional to diameter, Dp. A PAS2000CE (Photoelectric Aerosol Sensor manufactured by EcoChem) gave both size and composition-dependent responses. For diesel particles produced at high engine loads, the response was nearly proportional to Fuchs surface area. However, at lighter engine loads, the response dropped sharply with decreasing Dp. Light engine loads are associated with high fractions of volatile particles that may suppress the photoemission response. The secondary purpose of this study is to investigate the difference in charging rate between singlets (NaCl particles) and agglomerates (diesel particles) by using diffusion chargers. Agglomerates (diesel particles at engine load 75%) acquire more charge than singlets (NaCl particles) by 15 and 17% for LQ1-DC and EAD, respectively.

Real-time characterization of ultrafine and accumulation mode particles in ambient combustion aerosols

Abstract The diffusion charging sensor (DC), photoelectric aerosol sensor (PAS) and condensation particle counter (CPC) are real-time particle instruments that have time resolutions s and are suitable for field use. This paper shows how the relative fraction of nuclei mode particles (D⩽50 nm ) in ambient combustion aerosols can be determined, along with the coverage degree of the respective accumulation mode particles with a modal diameter of ∼100 nm . Main tools for interpretation are the diameter of average surface D Ave, S (obtained from CPC and DC measurements) and PAS/DC versus D Ave, S scatter plots. Compared to the scanning mobility particle sizer (SMPS), which is a standard instrument for aerosol particle size distribution measurements, the presented method has a limited accuracy, but is substantially faster. Additionally, it is experimentally less demanding than SMPS measurements, especially for field applications.